28-01-2012, 09:42 AM
ACKNOWLEDGEMENT
We, the aforementioned, from B.Tech. (Electrical Engineering) take great pleasure in presenting in presenting this report titled 'Study and Design of Power Transformer under Short-Circuit conditions'. We would like to take this opportunity to thank Dr. B.K. Lande, Head of the Department of Electrical Engineering, Veermata Jijabai Technological Institute, who permitted us to undertake this project.
We would like to convey our deepest appreciation for Dr. M. S. Panse from the Electrical Engineering Department, whose continued support and motivation, has allowed us to express ourselves and helped us to complete this project within the stipulated time. We would like to thank all the college officials for their co-operation.
A special vote of thanks goes out to Mr. Shekhar Vora, from Crompton Greaves Ltd., whose excellent guidance and valuable inputs, helped make this project a reality.
We would also like to express our deepest gratitude to all the people, who have directly or indirectly, influenced the completion of this project.
ABSTRACT
The project deals with the study of the practical power transformer as used in transformer industries and its design to successfully withstand severe short circuit conditions.
Highlighting the basics of a power transformer (like emf equation, windings, core), we designed a practical transformer using design specifications provided to us by Crompton Greaves Ltd. like MVA rating of transformer, core diameter, % impedance restrictions, core and copper loss limits, constraint on the price of the transformer.
After having successfully designed a power transformer according to the given specifications and meeting the loss, impedance and price constraints, we proceeded to analyzing the short circuit withstand capacity of the designed transformer.
The short circuit analysis involves calculation of first peak of short circuit current, calculation of radial and axial stresses on the windings and on the insulation blocks placed in between windings.
The radial and axial stresses calculated were found to be well within the stipulated limits given in the design problem and hence, we conclude that our transformer has been successfully passed to withstand short circuit conditions.
CHAPTER 1
INTRODUCTION (1)
In the recent development of the power systems, increase in the power plant capacity has been a major achievement in the power industry. This power is transmitted at a voltage which is much higher than the generated voltage as the power loss at high voltage and low current is lesser than that at low voltage and high current. Hence, it is necessary to step up the voltage level of the generated voltage to a much higher value for transmission purposes in order to avoid extensive power loss. For this purpose, power transformers are widely used in the industry.
Addition of more generating capacity and interconnections in the power systems have contributed to an increase in the short circuit capacity of networks, making the short circuit duty of power transformers more severe. Failure of transformers due to short circuits is an area of major concern for transformer manufacturers and consumers. There are continuous efforts by manufacturers and consumers to improve the short circuit withstand performance of transformers.
The short circuit strength of a power transformer enables it to survive through fault currents due to external short circuits in a power system network. An inadequate strength may lead to a mechanical collapse of windings, deformation/damage to clamping structures, or an electrical fault in the transformer itself. The internal faults initiated by the external short circuits are dangerous as they may involve blow-out of bushings, fire hazards and may eventually lead to bursting of the entire transformer structure.
In recent short circuit tests on power transformers greater than 100 MVA, it has been seen that increase of short circuit inductance beyond 1% has caused significant deformation in windings. A much stricter control on the variations in materials and manufacturing processes will have to be exercised to avoid looseness and winding movements. Hence, the causes of internal short-circuits in the transformer are inherently related to the design parameters of the transformer.
Thus, it is always desirable to design a transformer for normal working conditions and then make variations to it in order to account for the forces developed on transformer winding during short-circuit condition.
2.1 WINDINGS
The transformer consists of two coils called „windings‟ which are wrapped around a core. The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other. The winding that is connected to the source is called the „primary winding‟. The winding that is connected to the load is called the „secondary winding‟.
The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn.
The insulation system must be designed to withstand the effects of lightning strikes and switching surges to which the transformer is subjected, in addition to the normal operating voltages. A further requirement of the insulation system is that it must withstand the environmental conditions to which it is exposed, such as moisture, dust etc. A variety of techniques and materials are employed to achieve the necessary performance characteristics of the insulation system.
